As a precaution against malfunctions, boats and ships or the like which use wireless communication apparatuses are equipped, for example, with two wireless communication apparatus. Both apparatuses are turned on, one predetermined unit of which is used as the main wireless communication apparatus, and the other is held in reserve as a standby unit.
Under these circumstances, the signals transmitted from the one main wireless communication apparatus are received by the other spare wireless communication apparatuses i.e. the other standby apparatuses.
A technique for switching between the currently used apparatus (this is called a current apparatus) and the standby apparatus during the use of such wireless communication apparatuses has been disclosed, for example, in the following patent document 1.    Patent Document 1: Laid open Japanese Patent Publication (unexamined) No. 296719/1989
When two wireless communication apparatuses, for example, are provided in a limited space such as in boats and ships or the like, the antennas for the two wireless communication apparatuses must be set up adjacent to each other. In such cases, the signal strength from the main wireless communication apparatus will be too strong for the other adjacent standby wireless communication apparatus, and the receiver of the standby wireless communication apparatus may therefore become damaged. Even if the receiver is not damaged, there have been problems in that the radio signals from the main wireless communication apparatus act as interference on the adjacent standby wireless communication apparatus.
Furthermore, even when a main wireless communication apparatus, i.e. a current wireless communication apparatus, and at least one more standby wireless communication apparatus are operated on unequal frequencies separately from each other, there have been problems in that a noise or voice sound is put out from the speaker of the standby apparatus due to spurious signals included in the signals transmitted from the main wireless communication apparatus.
However, no technique for eliminating the problems described above has been disclosed in the above-mentioned Patent Document 1 or the like, and in actuality users must resort to manually turning off the standby wireless communication apparatus, lowering the volume of the standby wireless communication apparatus, or the like, which is inconvenient.